1. Field of the Invention
The present invention relates to internetwork data transfer and, more particularly, to apparatus for bridging data across a non-compatible network interface and a system which implements the same.
2. Description of the Related Art
Various communication media provide channels or paths which link various data processing equipment. Communication media or networks that share a common communication channel are sometimes referred to as shared-channel networks or multi-access media. Within multi-access media, signals launched to/received from any one station (e.g., a data processor) may be directed to or received from a variety of other stations or processors within the network. Examples of muiti-access media are local area networks (LANs), wide area networks (WANs), metropolitan area networks (MANs), satellite networks and packet radio networks, etc.
Local area networks (LANs) provide a communication medium that is shared among a plurality of attached stations, e.g., microcomputers, office machines, etc. Local area networks utilize what is commonly referred to as a "layered" protocol to transmit data blocks on a local shared bus. The data are transmitted with explicit addresses that are recognized by the destination station for delivery. A StarLan local area network is an example of a widely used local area network which utilizes a distributed (or medium access) protocol to regulate station or processor access to the common transmission medium (i.e., the local area network bus). A set of cooperating adapters attaches each station to the local area network (i.e., a local area network interface). Through the adapters, the appropriate "layered" protocol is provided to access the network, to buffer data for exchange and to physically interface with other LAN-resident stations.
Wide area networks (WANs) define an extended or wide transmission architecture for communicating, for example, nationally or internationally. As with local area networks, wide area networks utilize a layered protocol to accomplish network communication over a shared medium. FIG. 1 illustrates a common, layered, wide area network architecture based on the well known open systems interconnection (OSI) model. The illustration of FIG. 1 exemplifies a logical connection between data applications X and Y with one intermediate node. The layers at each station (i.e., data application) are arranged in a hierarchical configuration, in which the lower layers (i.e., network, data link and physical layers) function to provide a physical connection between users or processes, and the upper layers (i.e., application, presentation, session and transport layers) provide actual data exchange between processes and/or users.
(X.25) designates the most widely currently used wide area network standard, the protocol of which is based on the Comite Consultatif Internationale Telegraphique et Telephonique (CCITT) definition of the lower three layers of the OSI model. The (X.25) protocol regulates access and connection of data terminals, computers and other equipment, i.e., data terminal equipment (DTE), to the packet-switched WAN via data communication equipment (DCE). The (X.25) packet layer (i.e., the network layer of the OSI model) essentially provides a virtual circuit between processes across the wide area network. FIG. 2 shows an (X.25) WAN with a direct connection from data terminal equipment A to data terminal equipment B, and a direct connection from data terminal equipment A to data terminal equipment C. Data transmitted to/from data terminal equipment across the (X.25) WAN must be arranged with the (X.25) WAN protocol.
While communication between data terminals interconnected within a local area network (LAN) or within a wide area network (WAN) is extremely useful, the differing local area network and wide area network protocol prohibit LAN/WAN communication without some type of interface. In other words, data formed within LAN-resident equipment is not readily interpreted by a wide area network controller. To communicate across a wide area network, LAN-resident equipment must utilize some type of bridge or packet assembler/disassembler (PAD) to translate local area network data to the wide area network format and back. For example, dedicated diagnostic equipment designed to communicate with a target processor co-resident within a first local area network is unable to communicate with a second target processor resident within a second separate local area network, i.e., across a communication medium connecting the first and second local area networks, without some type of bridging or interface means.
Therefore, a need exists for apparatus which can bridge or link, for example, a dedicated diagnostic, LAN-resident processor, to a LAN-resident target processor across a non LAN-compatible wide area network. More particularly, a need exists for an interface or bridging apparatus which can adjust data transferred from/to a first LAN-resident processor to/from a second LAN-resident processor (compatible with the first) across a wide area network (WAN) while preserving a local area network interface with each of the first and second LAN-resident processors.